tbourner Posted November 9, 2006 Share Posted November 9, 2006 When you take the blocks off the barge the barge goes up in the water AND the water level drops!!! When you drop the blocks in the water the water level rises along with the barge - so it ends up higher than it did before. I don't know how much by though. Quote Link to comment Share on other sites More sharing options...
Ewen Posted November 9, 2006 Author Share Posted November 9, 2006 I'm assuming that you can answer this fully with the inf given, but I'm getting stuck on the fact that while in the barge the blocks will be displacing a mass of water equal to thier own mass (i.e. a greater volume than their own) but once in the water they will sink and so displace a volume of water equal to their own. So the blocks displave more water while they are still in the barge, and less when they are in the water. Also the barge displaces less water when the blocks are off loaded. Holy moly - thats it. The water level will drop. Because both the barge and the blocks are displacing less water. right argument, right answer Quote Link to comment Share on other sites More sharing options...
Mike Posted November 9, 2006 Share Posted November 9, 2006 Yeah, like I said.... Quote Link to comment Share on other sites More sharing options...
tbourner Posted November 9, 2006 Share Posted November 9, 2006 So basically concrete is more massive than water, so the weight of the block displaces more water than the volume of the block? Quote Link to comment Share on other sites More sharing options...
Ark Posted November 9, 2006 Share Posted November 9, 2006 In order to float, an object has to displace a volume of water equal to or greater than its own weight. Lets say that 1 metre cubed of concrete = 1 tonne = 1 metre cubed of water, then the water level will stay the same. However, the chances are that concrete is denser (hence it sinks) than water so Quote Link to comment Share on other sites More sharing options...
peter richards Posted November 9, 2006 Share Posted November 9, 2006 see i was correct all the time Quote Link to comment Share on other sites More sharing options...
Pete Posted November 9, 2006 Share Posted November 9, 2006 I need more info. How big is this barge? What's the surface area of it? What's the captains name? Is there a passing cormarant sitting on the aerial mast? What colour is it? Quote Link to comment Share on other sites More sharing options...
Ark Posted November 9, 2006 Share Posted November 9, 2006 Thinking back, didn't the great minds of British naval architects come up with a way to make concrete float? They used that during D-Day to float in these great big floating blocks of concrete, strung them all together to make pontoons to offload the big ships. There are still a couple of these to be seen off Weymouth, if you don't believe me. So, going back to my original argument, if these are floating blocks, then the water level will rise, due to water displacement. But then I guess there would have been no point in putting them on the barge in the first place... Quote Link to comment Share on other sites More sharing options...
Ewen Posted November 9, 2006 Author Share Posted November 9, 2006 Tricky isnt it. Basically about relative densities. Boats float because their relative density is less than water. The 300 tonne barge displaces 300 tonnes of fresh water, or 300 cubic metres of fresh water. Take 50 tonnes of concrete off the boat (assume the concretes dissappeared for a minute) and it now weighs 250 tonnes and displaces 250 cubic metres of water. The water level in the tank will fall. Now put the concrete blocks back, but into the water, not the boat. The concrete blocks weigh 10 tonnes each, but as their relative density is (approx) 2.5 times greater than water, the volume of water they displace is about 2.5 times less than the equivalent weight of a floating boat will displace. So although there is the same weight within the tank, there is less volume displacement, so the water level will fall. The cormorants name was Kevin. Quote Link to comment Share on other sites More sharing options...
tbourner Posted November 9, 2006 Share Posted November 9, 2006 Am I correct in saying that when you unload the barge the water level will drop by HALF that of the displacing mass removed? So 50 tonnes comes off, the water drops by 25 tonnes (25 cubic metres). 50 tonnes of concrete in the pool (approx. 20 cubic metres) means the water level has dropped by 5 cubic metres? Surely though the mass of the barge (now 250 tonnes) has an effect too? Does the water level drop by 50/250 (1/5) when the blocks are removed and not half like I said? I'm so confused - I can see ships!! Quote Link to comment Share on other sites More sharing options...
Thorin Posted November 9, 2006 Share Posted November 9, 2006 The cormorants name was Kevin. That's a terrible name for a cormorant. Quote Link to comment Share on other sites More sharing options...
Whitesupraboy2 Posted November 9, 2006 Share Posted November 9, 2006 see i was correct all the time I'll think you'll find technically i was the first with the correct answer Quote Link to comment Share on other sites More sharing options...
Pete Posted November 9, 2006 Share Posted November 9, 2006 The cormorants name was Kevin. Well if you'd said that in the first place I'd have got it right! Quote Link to comment Share on other sites More sharing options...
Digsy Posted November 9, 2006 Share Posted November 9, 2006 Thinking back, didn't the great minds of British naval architects come up with a way to make concrete float? They used that during D-Day to float in these great big floating blocks of concrete, strung them all together to make pontoons to offload the big ships. You're talking about "Mulberry". Two complete harbours that were pre-fabricated from hollow concrete blocks and sailed across the channel and then filled with water until they settled on the seabed just off the French coast. You could make a boat out of depleted uranium if you made its hull area big enough and its sides high enough so that by the time it had settled deep enough to diaplace a volume of water equal to its mass, the water wasn't coming over the sides. If you want a really whacky WW2 story, there was a short-lived plan to build HMS Habercook: A gigantic aircraft carrier made out of a mixture of sawdust and ice. A frozen sawdust and ice slurry is called Piecrete, and it is strong enough to be structural, melts at an incredibly slow rate and - being less dense than water - floats. Quote Link to comment Share on other sites More sharing options...
Ewen Posted November 9, 2006 Author Share Posted November 9, 2006 You're talking about "Mulberry". Two complete harbours that were pre-fabricated from hollow concrete blocks and sailed across the channel and then filled with water until they settled on the seabed just off the French coast. You could make a boat out of depleted uranium if you made its hull area big enough and its sides high enough so that by the time it had settled deep enough to diaplace a volume of water equal to its mass, the water wasn't coming over the sides. If you want a really whacky WW2 story, there was a short-lived plan to build HMS Habercook: A gigantic aircraft carrier made out of a mixture of sawdust and ice. A frozen sawdust and ice slurry is called Piecrete, and it is strong enough to be structural, melts at an incredibly slow rate and - being less dense than water - floats. Read about this...amazing story. Theres quite a few concrete yachts around the world too. Quote Link to comment Share on other sites More sharing options...
tbourner Posted November 9, 2006 Share Posted November 9, 2006 You could make a boat out of depleted uranium if you made its hull area big enough and its sides high enough so that by the time it had settled deep enough to diaplace a volume of water equal to its mass, the water wasn't coming over the sides. Is that the most massive thing we know? If you want a really whacky WW2 story, there was a short-lived plan to build HMS Habercook: A gigantic aircraft carrier made out of a mixture of sawdust and ice. A frozen sawdust and ice slurry is called Piecrete, and it is strong enough to be structural, melts at an incredibly slow rate and - being less dense than water - floats. How slowly are we talking? Quote Link to comment Share on other sites More sharing options...
Digsy Posted November 9, 2006 Share Posted November 9, 2006 Is that the most massive thing we know? Its one of the densest materials we know of. Not sure if it is the densest. How slowly are we talking? Not sure, but I know its a low slower than regular ice. The story goes that a navy high-up (might have been Mountbatten) pitched the idea to Churchill while he was taking a bath. He lobbed a lump of Pyecrete into the tub and it didn't melt all the time they were talking. I did a web search and found this link which saya that a prototype was made in Canada that floated throughout the summer. It did have a refirgeration plant on board, though. Quote Link to comment Share on other sites More sharing options...
peter richards Posted November 9, 2006 Share Posted November 9, 2006 talking of dense ewen lol:p Quote Link to comment Share on other sites More sharing options...
martin_a Posted November 9, 2006 Share Posted November 9, 2006 I think the issue is that mass and weight are being confused. In the water the blocks disperse their mass in water, i.e. a concrete block sized section of water while floating in the boat they displace their weight. As they weigh a lot but aren't very massive the water level drops. Its like Neil armstrong bouncing around on the moon, he is still Neil Armstrong, he takes up just as much space but the act of gravity on him means he weighs very little allowing him to bounce around - weightlessness. Weight is just another measure of gravity on your mass. m. Quote Link to comment Share on other sites More sharing options...
KaoriFan Posted November 9, 2006 Share Posted November 9, 2006 yes, Lord Mountbatten was very much in favour of Piecrete ships (aircraft carriers), and even took it to the Allies Higher Command to put the case forward. Can't remember the name of the US Admiral, but Mountbatten took his revolver out in the room and shot the piecrete cube, it didn't splinter or anything, stayed totally stable. they were wanting to build a fleet of aircraft carriers, but other events in WWII overtook this. Quote Link to comment Share on other sites More sharing options...
Rob Posted November 9, 2006 Share Posted November 9, 2006 So Lord Mountbatten was pitching his idea to Churchill, while Churchill was in the bath? I mean, I'm all for having a close team leading the country through a war, but really??? Quote Link to comment Share on other sites More sharing options...
garethr Posted November 10, 2006 Share Posted November 10, 2006 Thinking back, didn't the great minds of British naval architects come up with a way to make concrete float? They used that during D-Day to float in these great big floating blocks of concrete, strung them all together to make pontoons to offload the big ships.The US built concrete cargo ships in WW11. EDIT: In WW1 as well, apparently http://www.concreteships.org/ Quote Link to comment Share on other sites More sharing options...
Ark Posted November 10, 2006 Share Posted November 10, 2006 You've got to admit, that is COOL! Quote Link to comment Share on other sites More sharing options...
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